Liquid crystal materials, mixtures and devices

ABSTRACT

Liquid crystal compounds of formula I may be used by themselves or they may be mixed with other liquid crystal compounds to give useful liquid crystal mixtures which may then be used in liquid crystal devices. The materials exhibit smectic mesophases and may therefore be used in ferroelectric, ferrielectric, antiferroelectric, themochromic and electroclinic devices. They may also be used as long pitch materials.

This application is a 371 of PCT/GB94/01231, filed Jun. 7, 1994.

This invention relates to novel liquid crystal compounds, liquid crystalmaterials containing them and their inclusion in liquid crystal devices.

This application is a 371 of PCT/GB94/01231, filed Jun. 7, 1994.

This invention relates to novel liquid crystal compounds, liquid crystalmaterials containing them and their inclusion in liquid crystal devices.

Liquid crystals can exist in various phases. In essence there arevarious classes of liquid-crystalline material, each possessing acharacteristic molecular arrangement. For example there are nematic,cholesteric and smectic phases. A wide range of smectic phases exists,for example smectic A and smectic C. Some liquid crystal materialspossess a number of liquid crystal phases on varying the temperature,while others have just one phase. For example, a liquid crystal materialmay show the following phases on being cooled from the isotropic phase:isotropic - nematic - smectic A - smectic C - solid. If a material isdescribed as being smectic A then it means that the material possesses asmectic A phase over a useful working temperature range.

Devices containing ferroelectric liquid crystal mixtures exhibit fastswitching times (faster than 100 μs), Clark and Lagerwall, Appl. Phys.Lett., 36, 89, 1980. They can be bistable which means that they can bemultiplexed at high levels using a line-at-a-time fast scan technique.Ferroelectric materials continue to receive a large amount ofinvestigative attention due to their application in high resolution flatpanel displays. An important feature of devices containing liquidcrystalline materials is that they should exhibit a fast response time.The response time is dependent on a number of factors, one of thesebeing the spontaneous polarisation, denoted Ps (measured in nC cm⁻²). Byadding a chiral dopant to the liquid crystalline mixture the value of Pscan be increased, thus decreasing the response time of the device.Ferroelectric smectic liquid crystal materials, which can be produced bymixing an achiral host and a chiral dopant, use the ferroelectricproperties of the tilted chiral smectic C, F, G, H, I, J and K phases.The chiral smectic C phase is denoted S_(c).sup.• with the asteriskdenoting chirality. The S_(C).sup.• phase is generally considered to bethe most useful as it is the least viscous. It is desirable that thematerial should exhibit a long pitch nematic (denoted N.sup.•) andS_(A).sup.• phase at temperatures above the chiral smectic phase inorder to assist surface alignment in a device containingliquid-crystalline material. Ferroelectric smectic liquid crystalmaterials should ideally possess the following characteristics: lowviscosity, controllable Ps and an S_(C) phase that persists over a broadtemperature range, which should include ambient temperature, andexhibits chemical and photochemical stability. Materials which possessthese characteristics offer the prospect of very fast switching liquidcrystal containing devices. Some applications of ferroelectric liquidcrystals are described by J.S. Patel and J.W. Goodby in Opt. Eng., 1987,26, 273.

Other smectic phases exhibit exploitable characteristics. For examplethe electroclinic effect, first described by S. Garoff and R. Meyer,Phys. Rev. Lett., 38, 848, (1977), usually occurs in the smectic Aphase. Unlike ferroelectric devices, the liquid crystal material inelectroclinic devices is not bistable. The liquid crystal directorwithin an EC device responds almost linearly to an applied electricfield. Electroclinic devices are suitable for various applicationsincluding spatial light modulators. UK Patent Application GB 2 244 566 Adescribes an example of an electroclinic device.

Chandani et al., Jpn. J. Appl. Phys., 27, L 729, 1988; Jpn. J. Appl.Phys., 28, L 1261, 1989; Jpn. J. Appl. Phys., 28, L 1265, 1989, firstdescribed the antiferroelectric effect which is a tri-stable switchingstate occurring in a liquid crystal phase designated as SmC_(A).sup.•.For example, when ferroelectric layers are stacked so that thepolarisation vectors in sequential layers oppose one another then anantiferroelectric phase is obtained.

For a review of thermochromism in liquid crystals see J.G. Grabmaier in`Applications of Liquid Crystals`, G. Meier, E. Sackmann and J.G.Grabmaier, Springer-Verlag, Berlin and New York, 1975, pp 83-158.

It is not usual for a single compound to exhibit all of the propertiesoutlined in the preceeding text, hence ferroelectric smectic liquidcrystal materials generally consist of a mixture of compounds which whenmixed together induce a chiral tilted smectic phase. Some of thecompounds which are added to such a mixture are described as additives.Chiral dopants come under the term additive and are added to the liquidcrystalline mixture in order to induce the smectic mixture to becomechiral smectic and to induce a Ps in the material, or if the materialalready possesses a Ps then the introduction of a chiral dopant shouldresult in a change of value for Ps. The chiral dopant provides thenecessary Ps component required for ferroelectric switching.

The host is generally a material that shows a smectic phase (preferablytilted smectic, especially S_(C)) without being chiral. The dopant is orcontains at least one optically active compound, without necessarilyshowing a smectic phase, although it is preferred if the dopant doesitself show a smectic phase. The dopant when mixed with the host resultsin the mixture becoming chiral and induces a Ps.

Some chiral compounds show smectic phase(s) and are thereforetheoretically suitable as both hosts and dopants. This means that it maybe possible to have a switchable ferroelectric material made up of asingle compound. In practice this is rarely achieved because this typeof compound is often very viscous and so has slow response times.

PCT patent application WO 86/00087 describes a series of opticallyactive liquid crystal compounds that contain the chiral groups: ##STR2##where X represents Cl, CN or CH₃ and R₁ and R₂ represent the residue ofthe molecule. All of the compounds described necessarily contain thephenyl-pyrimidine group, ##STR3## as the mesogenic unit. The pyrimidinering is said to be particularly beneficial in the short moleculesdescribed as its molecular configuration increases intermoleculardistances in the bulk, thus reducing the viscosity. Among the manycompounds described are: ##STR4## These two compounds do not showsmectic phases by themselves.

According to this invention there is provided compounds having a generalFormula I ##STR5## in which R is selected from alkyl, alkoxy or alkenyland may contain 1-20 carbon atoms; A, B. C, D are independently selectedfrom phenyl, cyclohexyl, pyridyl, pyrimidyl; A may also be dioxanyl,napthyl; b is independently selected from 0, 1 or 2; c is independentlyselected from 0 or 1; a and d are both equal to 1; K, L, M, N areindependently selected from the halogen group; (k), (1), (m), (n), areindependently 0, 1, 2, 3, or 4; X is selected from OCO, COO, OCH₂, CH₂0, CH₂ CH₂ ; Y is selected from OCO, COO, OOC, OCH₂, CH₂ 0, CH₂ ; J isan end group of Formula II which contains a chiral centre ##STR6## whereZ is selected from halogen, C₁ --C₃ alkyl chain, CN, CF₃, CHF₂, H; R₁ isa linear or branched alkyl group containing 1-15 carbon atoms or H, W isa linear or branched alkyl group containing 1-5 carbon atoms or H;excluding where R is alkoxy, A=B=D=phenyl, K=F, (k)=2, (l)=(n)=O, b=1c=O, X=OOC, Y=OOC, Z=Cl, R₁ =CH₂ CH(CH₃)₂ W=H;

provided that the total of (k)+(l)+(m)+(n) does not equal 0 and that thetotal of a+b+c+d is not greater than 4.

Preferably R is C₃₋₁₂ even more preferably C₅₋₉.

Preferably A, B, D are phenyl.

Preferably b=1 and c=0.

Preferably K, L, N are independently of each other F or H.

Preferably (k), (1), (n) are independently of one another 0, 1 or 2;even more preferably (k) may be 0, 1 or 2 and (l) and (n) may beindependently of each other 0 or 1.

Preferably X is OCO or OCH₂ or CH₂ O; even more preferably X is OCO.

Preferably Y is OOC or OCH₂.

Preferably Z is F or C₁₋₃ or H; even more preferably Z is F or CH₃ or H.

Preferably R₁ contains 1-9 carbon atoms or H; even more preferably R₁ isC₁₋₆ or H. Preferably W is C₁₋₃ or H; even more preferably W may contain1 carbon atom or be H.

The compounds of formula I are further characterised in that they may beused as host materials and dopants. They possess a SC phase thereforemeasurements on them to deduce, for example Ps, may be done in theirpure form.

Compounds of formula I may be used as optically active components offerroelectric smectic liquid crystal mixtures i.e. as chiral dopants.When used as components of such mixtures compounds of formula I,particularly the preferred compounds referred to above, may offer thefollowing advantages.

i/ They may show a high spontaneous polarisation coefficient (Ps). Thismay conveniently be expressed in terms of the extrapolated Ps i.e. thePs of the mixture extrapolated to 100% of the compound of formula I.This means that quite a small amount of the compound of formula I needbe included in the mixture.

ii/ They may induce the appearance of chiral smectic phases in themixture having a very long helical pitch. This may more conveniently beassessed by measuring the chiral nematic N.sup.• pitch they induce whenmixed with a nematic liquid crystal material. A long pitch is oftendesirable as in some ferroelectric smectic liquid crystal devices thepitch should be as close as possible to the spacing of the electrodes,and in practice the difficulty of manufacture increases with decreasingelectrode spacing.

iii/ Chiral smectic mixtures containing them may show S_(C).sup.• phaseswhich persist over a wide temperature range, including room temperature.

iv/ They are compatible with many hosts and additives for example thosediscussed below.

v/ They may offer the possibility of very high switching speeds, whichis of advantage in for example video screen type applications. This ispartly due to i/ above in that many known chiral dopants are viscous andcause mixtures containing them to be viscous. The good Ps induced bycompounds of formula I means that relatively little need be used andhence there is little adverse effect on viscosity.

vi/ It is often possible to obtain compounds of formula I in both (R)and (S) enantiomeric forms as both enantiomeric forms of the startingmaterial may be available. This makes the pitch of mixtures containingthem particularly easy to `compensate` (see below) by includingopposite-twisting enantiomers of the compounds of formula I in themixture.

A ferroelectric smectic liquid crystal mixture according to theinvention contains at least one compound of formula I. Typically if thecompound is present as a dopant the mixture will contain 1-20% by weightof the compound of formula I, eg around 10% or less. Generally the Ps ofthe mixture is proportional to the amount of chiral dopant present.

The mixture should contain one or more compounds which either separatelyor together show an Sc phase. Such compounds are known as smectic hosts.

A large number of classes of compounds which may be used as smectichosts, and some examples of suitable classes are discussed below.Compounds of formula I may be mixed with a wide range of hosts, forexample smectic hosts to form a useful liquid crystal composition. Suchcompositions can have a range of Ps values. Compounds of formula I maybe mixed with one or more of the types of hosts VIII-XIII. Thesedifferent types of hosts may be mixed together to which the compound ofgeneral formula I may also may be added.

Typical hosts include:

The compounds described in PCT/GB86/0040, eg of formula VIII ##STR7##where R₁ and R₂ are independently C₃ -C₁₂ alkyl or alkoxy.

The fluoro-terphenyls described in EPA 8430494.3 and GBA 8725928, eg offormula IX ##STR8## where R₁ and R₂ are independently C₃ -C₁₂ alkyl oralkoxy, x is 1 and F may be on any of the available substitutionpositions on the phenyl ring specified.

The difluoro-terphenyls described in GBA 8905422.5, eg of formula X##STR9## where R₁ and R₂ are independently C₃ --C₁₂ alkyl or alkoxy.

The phenyl-pyrimidines described in WO 86/00087, eg of formula XI##STR10## including those compounds where R₁ is C₃ -C₁₂ alkyl and R₂ isgiven by the general formula (CH₂)_(n) --CHXCH₂ CH₃, where n is 1 to 5and X is CN or Cl.

The compounds described by R. Eidenschink et. at. inCyclohexanederivative mit Getilteneten Smektischen Phasen at the 16_(th)Freiberg Liquid Crystal Conference, Freiberg, Germany, p8. Availablefrom E. Merck Ltd., Germany, eg of formula XII. ##STR11## includingthose compounds where R₁ and R₂ are independently C₁ -C₁₅ alkyl.

The difluoro-phenyl pyrimidines described at the ₂ nd InternationalSymposium on Ferroelectric Liquid Crystals, Go bteborg, Sweden, June1989 by Reiffenrath et. al., eg of formula XIII ##STR12## includingthose compounds where R₁ and R₂ are independently C₃ --C₉ alkyl.

These hosts allow the possibility of sc mixtures showing an SC phasepersisting over a wide temperature range including room temperature, andalso an SA phase at a temperature above the SC, to assist in thealignment of the liquid crystal material.

The compounds of formula I may also may be used as host materials.

Additives in such a mixture may serve a number of functions. One suchfunction is as pitch compensators. Pitch compensation is the inclusionin the ferroelectric smectic mixture of two or more compounds whichinduce the appearance of helical smectic phases of opposite twist sense.In such a case the compounds will unwind the helical phase induced bythe other. This may be used to produce a long pitch helical smecticphase, and by the controlled use of appropriate quantities of the twocompounds the pitch of the mixture may be closely controlled.

In mixtures according to the invention, pitch compensation may beachieved conveniently by using opposite-twisting compounds of formula Ior by using different compounds from formula I.

The compounds of formula I may be advantageously used in ferroelectricliquid crystal displays, electroclinic displays and as long pitchmaterials.

The intention will now be described by way of example only withreference to the accompanying drawings of which:

FIG. 1 describes a synthetic route for the preparation of compounds3-13.

FIG. 2 describes a synthetic route for the preparation of compounds15-19.

FIG. 3 describes a synthetic route for the preparation of compounds22-23.

FIG. 4 describes a synthetic route for the preparation of compounds24-25.

FIG. 5 describes a synthetic route for the preparation of compounds27-31.

FIG. 6 describes a synthetic route for the preparation of compounds 33.

FIG. 7 describes a synthetic route for the preparation of compounds 361438, 40-43.

FIG. 8 describes a synthetic route for the preparation of compounds 44and 46.

FIG. 9 describes a synthetic route for the preparation of compounds 45and 47.

FIG. 10 describes a synthetic route for the preparation of compounds52-53.

FIG. 11 describes a synthetic route for the preparation of compound 48.

FIG. 12 describes a synthetic route for the preparation of compound 49.

FIG. 13 describes a synthetic route for the preparation of compounds 50,54 and 55.

FIG. 14 illustrates a liquid crystal device.

FIG. 15 is a graph of tilt angle (°) versus temperature (°C.) forcompound 49.

FIG. 16 is a graph of tilt angle (°) versus temperature (°C.) forcompound 50.

FIG. 17 is a graph of temperature (°C.) versus pitch (μm) in thecholesteric phase close to the infinite point for compound 49.

FIG. 18 is a graph of spontaneous polarisation (nC/cm²)versustemperature (°C.) for compound 49. The plotted points are for threeseparate runs.

Reagents used in the synthetic route of FIGS. 1-13 are shown in thecorresponding Schemes 1-13.

Difluorophenol (1) and 1-bromo-3-fluorophenol (2) were purchased fromFluorochem Ltd.

2-fluoro-4-hydroxybenzoic acid was obtained from Merck Ltd.

(S)-2-fluoro-octanoic acid (20) and (S)-2-fluoro-octanol (21) wereobtained from E. Merck.

(S)-4-(2,6-dimethylheptylcarbonyloxy)benzoic acid (32) is a standardintermediate. ##STR13## Compound 3: 1,2-difluoro-3-octoxybenzene

A solution of 1-bromo-octane (25.1g, 0.13mol) in acetone (30ml) wasadded to a stirred, refluxing mixture of 2,3-difluorophenol (1) (14g,0.108mol) and potassium carbonate (20.0g, 0.144mol) in acetone (300ml).The stirred mixture was heated under reflux for 24hr (or until theanalysis revealed a complete reaction). The potassium carbonate wasfiltered off and most of the acetone removed in vacuo. The residue wasdissolved in ether, water was added and the layers separated. This wasfollowed by a second extraction of the aqueous layer with ether. Thecombined organic layers were washed with water, 5% sodium hydroxide,water and dried (MgSO4). The solvent was removed under vacuum. The crudeproduct was distilled to give a colourless oil.

Yield=17.1 g, (65%)

bp=78°-81° C. (0.4Nm⁻²)

Compound 4:

1-bromo-3-fluoro-4-octoxybenzene

This was prepared using a similar method to that described for compound3. The crude product was distilled. Quantities: 4-bromo-3-fluorophenol(10.0 g, 0.052 mol), 1-bromo-octane (12.0 g, 0.062 mol), potassiumcarbonate (14.5g, 0.105mol).

Yield=11.16 g (71%)

bp 112°-114° C. (1.3Nm ⁻²)

Compound 5:

2, 3-difluoro-4-octoxyphenylboronic acid

A solution of 1,2-difluoro-4-octoxybenzene (6.74g, 0.029 mol) (3), indry THF (100 ml) was cooled to -78° C. and n-butyllithium (3.2ml, 10M inhexane, 0.032mol) was added dropwise. The reaction mixture wasmaintained under these conditions for 2.5hr and then a solution oftri-isopropylborate (10.91 g, 0.058 mol) in dry THF (30 ml) was addeddropwise at -78° C. The reaction mixture was allowed to warm to roomtemperature overnight and then stirred for 1 h with 10% HCl (30ml). Theproduct was extracted into ether (twice), and the combined etherealextracts were washed with water and dried (MgSO₄). The solvent wasremoved in vacuo to give a colourless solid.

Yield=8.21 g, (98%)

Compound 6:

3-fluoro-4-octoxyphenylboronic acid

This was prepared using a similar method to that described for compound5. Quantities: 1-bromo-3-fluoro-4-octoxybenzene (15 g, 0.05 mol),n-butyllithium (5 ml, 10M in hexane, 0.05 mol), tri-isopropylborate(18.81 g, 0.1 mol).

Yield =7.5g, (56%).

Compound 8:

4'-bromo-2.3-difluoro-4-octoxybiphenyl

A solution of 2,3-difluro-4-octoxyphenylboronic acid (5) (8.29 g, 0.029mol) in dimethoxyethane (40 ml) was added to a solution of4-bromoiodobenzene (6.80 g, 0.024 mol) andtetrakis(triphenylphosphine)palladium (0) (1.⁴ 8 g. 1.29 mmol) indimethoxyethane (40 ml) under nitrogen. To this, 2M sodium carbonate (60ml) was added. The stirred mixture was refluxed gently. Progress of thereaction was carefully monitored using tic until the 4-bromoiodobenzenehad reacted completely (usually 4-5hr). The layers were separated andthe aqueous layer once more extracted with ether. The combined organiclayers were washed with brine and dried (MgSO₄). The solvent was removedin vacuo and the residue purified by column chromatography (silicagel/petroleum fraction (bp 40°-60° C.) - ethyl acetate, 9:1) to give awhite solid which was recrystallised from pentane (-20° C.) to yieldcolourless crystals.

Yield=5.28 g. (61%)

mp 39 °-40° C.

Compound 9:

4'-bromo-3-fluoro-4-octoxybiphenyl

This was prepared using a similar method to that described for compound8. Quantities: 3-fluoro-4-octoxyphenyl boronic acid (7.0 g, 0.026 mol),4-bromoiodobenzene (5.88 g, 0.021 mol),tetrakis(triphenylphosphine)palladium (0) (0.8 g, 0.7 mol).

Yield=5.09g, (64%)

mp=42°-44° C.

Compound 10:

2,3-difluoro-4-octoxybiphenyl-4'-yl-boronic acid

This was prepared using a similar method to that described for compound5. Quantities: 4'-bromo-2,3-difluoro-4-octoxybiphenyl (8) (3.18g, 8.0mmol), n-butyllithium (3.2 ml, 2.5M/hexane, 8.0 mmol),tri-isopropylborate (3.01 g, 16.0 mmol).

Yield=2.9 g, (100%).

Compound 11:

3-fluoro-4-octoxybiphenyl-4'-yl-boronic acid

This was prepared using a similar method to that described for compound5. Quantities: 4'bromo-3-fluoro-4-octoxybiphenyl (4.95 g, 0.013 mol),n-butyllithium (5 ml, 2.5M/hexane, 0.013 mol), tri-isopropylborate (4.7g, 0.026 mol).

Yield =4.40 g, (98%)

Compound 12:

2,3-difluoro-4-octoxy-4'-hydroxybiphenyl

10% hydrogen peroxide (10.5 ml, 0.023 mol) was added dropwise to astirred, refluxing solution of ²,³ -difluoro-⁴-octoxybiphenyl-4'-yl-boronic acid (10) (2.9g, 8.0 mmol), in diethylether (40 ml). The stirred mixture was heated under reflux until ticshowed the reaction to be complete. The ether layer was separated andthe aqueous layer extracted with ether. The combined ethereal layerswere washed with water and dried. The solvent was removed. The crudeproduct was purified by flash chromatography (silica gel/petroleumfraction (bp 40°-60° C.) - ethyl acetate, 2:1) to give a white solidwhich was recrystallised from pentane/ethyl acetate mixtures.

Yield=2.1 g, (78%),

mp=112.5°-113.5° C.

Compound 13:

3-fluoro-4-octoxy-4'-hydroxybiphenyl.

This was prepared using a similar method to that described for compound12. Quantities: 3-fluoro-4-octoxybiphenyl-4'-yl-boronic acid (11) (4.40g, 0.012 mol), 10% hydrogen peroxide (16 ml, 0.036 mol).

Yield=3.78 g, (72%),

mp=126°-28° C.

Compound 15:

4-methoxycarbonyloxybenzoic acid

A solution of sodium hydroxide (15 g) in water (400 ml) was chilled to0° C. in ice. To this the 4-hydroxy benzoic acid (17.9 g, 0.130 mol) wasadded. Methylchloroformate (20 g, 0.212 mol) was added slowly to preventthe temperature from rising above 5° C. The reaction mixture was stirredat 0°-5° C. for 3 h during which time a white suspension graduallyformed. The pH was adjusted to 4-5 with addition of HCl/water (1:1). Thevoluminous precipitate was filtered off, washed with water andrecrystallised from ethanol (200 ml).

Yield=23.2g. (91%). mp=177°-178° C.

Compound 16:

2,3-difluoro-4-octoxybiphenyl-4'-yl

4-methoxycarbonyloxybenzoate.

To a solution of 4-methoxycarbonyloxybenzoic acid (1.50 g, 7.64 mmol)(15) and 2,3 -difluoro-4-octoxy-4'-hydroxybiphenyl (12) (2.55 g, 7.64mmol) in THF (60 ml) was added diethylazodicarboxylate (1.329 g, 7.64mmol) under an atmosphere of dry nitrogen. Triphenylphosphine (2.003 g,7.64 mmol), dissolved in dry THF (60 ml) was added slowly to theresulting stirred solution. The solution was stirred for 8 h at roomtemperature when the solvent was removed under reduced pressure to givean off-white solid. The product was purified by flash chromatography(silica gel/petroleum fraction (bp 40°-60° C.) - dichloromethane, 1:2).

Yield=3.05 g, (78%)

Phase Transitions (°C) K 76.0/99.8N 201.9 I

Between the K(crystalline) and N(nematic) phases the first temperatureis the point of crystallisation and the second temperature is themelting point.

Compound 17:

3-fluoro-4-octoxybiphenyl-4'-yl

4-methoxycarbonyloxybenzoate

This was prepared using a similar method to that described for compound16. Quantities: 4-methoxycarbonyloxybenzoic acid (15) (0.74 g, 3.79mmol), triphenylphosphine (0.995g, 3.79mmol). Yield=1.3 g, (69%)

Phase Transitions (°C.) K 88.1/112.0N 216.2 I

Between the K(crystalline) and N(nematic) phases the first temperatureis the point of crystallisation and the second temperature is themelting point.

Compound 18:

2,3-difluoro-4-octoxybiphenyl-4'-yl

4-hydroxybenzoate.

A suspension of 2,3-difluoro-4-octoxybiphenyl-4'-yl4-methoxycarbonyloxybenzoate (2.7 g, 5.27 mmol) (16) was stirred in amixture of ethanol (60 ml) and ammonia (60 ml, 35%) at room temperaturefor 8 h or until the conversion was shown to be complete. The volatilecomponents were removed in vacuo (<55° C.) to give a white powder whichwas further dried in vacuo.

Yield=2.34 g, (97%)

mp=145° C.

Compound 19:

3-fluoro-4-octoxybiphenyl-4'-yl

4-hydroxybenzoate

This was prepared using a similar method to that described for compound18 and the product was recrystallised twice from ethyl acetate/hexanemixtures. Quantities: 3-fluoro-4-octoxybiphenyl-4'-yl4-methoxycarbonyloxybenzoate (1.3 g, 2.63 mmol).

Yield=0.91 g, (79%).

Compound 22:

2,3-difluoro-4-octoxybiphenyl-4'-yl

4-((S)-2-fluorooctanoyloxy)benzoate.

To a solution of 2.3-difluoro-4-octoxybiphenyl-4'-yl 4-hydroxybenzoate(18) (0.5 g, 1.1 mmol), (S)-2-fluorooctanoic acid (20) (0.178 g, 1.1mmol) and N,N-dimethylaminopyridine DMAP! (0.03 g) in drydichloromethane (25 ml) was added dropwise, but quickly, a solution ofdicyclohexylcarbodiimide (0.25 g, 1.2 mmol) in dry dichloromethane (25ml) under dry nitrogen. The reaction mixture was stirred at roomtemperature for 3h. The dicyclohexylurea precipitate was removed byfiltration and the solvent removed under reduced pressure. The productwas purified by flash chromatography (silica gel/petroleum fraction (bp40-60)-dichloromethane, 1:2 initially, but gradually increasing thepolarity to 1:9).

Yield=0.39 g (59%).

Compound 23:

2,3-difluro-4-octoxybiphenyl-4'-yl

4-((S)-2-fluorooctyloxy)benzoate.

This was prepared using a similar method to that described for compound16. Quantities: (S)-2-fluorooctanol (21) (0.16 3g, 1.1 mmol),2,3-difluoro-4-octoxybiphenyl-4'-yl 4-hydroxybenzoate (0.50 g, 1.1 mmol)(18), diethylazodicarboxylate (0.192g, 1.1 mmol), triphenylphosphine(0.29 g, 1.1 mmol). Yield=0.31 g, (48%).

Compound 24:

3-fluoro-4-octoxybiphenyl-4'-yl

4-((S)-2-fluorooctanoyloxy)benzoate.

This was prepared using a similar method to that described for compound22. Quantities: (S)-2-fluorooctanoic acid (20) (0.165 g, 1.0 mmol),3-fluoro-4-octoxybiphenyl-4'-yl 4-hydroxybenzoate (19) (0.445 g, 1.0mmol), dicyclohexylcarbodiimide (0.232 g, 1.0 mmol),N,N-dimethylaminopyridine (0.03 g).

Yield=0.43 g (73%).

Compound 25:

3-fluoro-4-octoxybiphenyl-4'-yl

4-((S)-2-fluorooctyloxy)benzoate.

This was prepared using a similar method as for compound 16. Quantities:(S)-2-fluorooctanol (21) (0.153 g, 1.03 mmol),3-fluoro-4-octoxybiphenyl-4'-yl 4-hydroxybenzoate (0.450 g, 1.03 mmol),diethylazodicarboxylate (0.179 g, 1.03 mmol), triphenylphosphine (0.270g, 1.03 mmol).

Yield=0.25g9 (43%).

Compound 27:

2-fluoro-4-methoxycarbonyloxybenzoic acid.

This was prepared using a similar method to that described for compound15. Quantities: 2-fluoro-4-hydroxybenzoic acid (26) (0.97 g, 6.23 mmol),methylchloroformate (1.09 g, 11.48 mmol).

Yield=1.5 g, (89%).

mp =165°-166° C.

Compound 28:

3-fluoro-4-octoxybiphenyl-4'-yl

2-fluoro-4-methoxycarbonyloxybenzoate.

This was prepared using a similar method to that described for compound16. Quantities: 2-fluoro-4-methoxycarbonyloxybenzoic acid (27) (0.86 g.4.0 mmol), 3-fluoro-4-octoxy-4'-hydroxybiphenyl (13),diethylazodicarboxylate (0.7 g, 4.0 mmol)triphenylphosphine (1.05 g, 4.0mmol).

Yield=1.10 g, (53%).

Phase Transitions (°C.) K 62.0/86.2N 192.1 I

Compound 29:

3-fluoro-4-octoxybiphenyl-4'-yl

2-fluoro-4-hydroxybenzoate

This was prepared using a similar method to that described for compound18.

Quantities:

b 3-fluoro-4-octoxybiphenyl-4'-yl 2-fluoro-4-methoxycarbonyloxybenzoate(28) (1.0 g, 2.0 mmol).

Yield=0.88 g, (99%).

Compound 30:

3-fluoro-4-octoxybiphenyl-4'-yl

2-fluoro-4-((S)-2-fluorooctanoyloxy) benzoate.

This was prepared using a similar method to that described for compound22. Quantities: (S)-2-fluorooctanoic acid (20) (0.178 g, 1.1 mmol),3-fluoro-4-octoxybiphenyl-4'-yl 2- fluoro-4-hydroxybenzoate (29) (0.⁴ 98g, 1.1 mmol), dicyclohexylcarbodiimide (0.248 g, 1.2 mmol),N,N-dimethylaminopyridine (0.03 g).

Yield=0.50 g, (76%).

Compound 31:

3-fluoro-4-octoxybiphenyl-4'-yl

2-fluoro-4-((S)-2-fluorooctyloxy)benzoate.

This was prepared using a similar method to that described for compound16. Quantities: (S)-2-fluoro-octanol (21) (0.147 g, 1.0 mmol),3-fluoro-4-octoxybiphenyl-4'-yl 2-fluoro-4-hydroxybenzoate (29),diethylazodicarboxylate (0.172 g, 1.0 mmol), triphenylphosphine (0.260g, 1.0 mmol).

Yield0.35 g, (60%).

Compound 33:

2,3-difluoro-4-octoxybiphenyl-4'-yl

4-((S)-3,7-dimethyloctanoyloxy)benzoate.

This was prepared using a similar method to that described for compound22. Quantities: 4-((S)-2,6-dimethylheptylcarbonyloxy)benzoic acid (32)(0.320g, 1.1 mmol, 2,3-difluoro-4-octoxy-4'-hydroxybiphenyl (12) (0.366g, 1.1 mmol), dicyclohexylcarbodiimide (0.250 g, 1.21 mmol),N,N-dimethylaminopyridine (0.04g).

Yield=0.48 g, (71%).

Compound 36:

3-fluoro-4-octoxy-4'-bromo-2'-fluorobiphenyl.

This was prepared using a similar method to that described for compound8. The product is a low melting solid and was used without furtherpurification. Quantities: 3-fluoro-4-octyloxyphenylboronic acid (6)(6.83 g, 0.026 mol), 1-bromo-3-fluoro-4-iodobenzene (6.92 g, 0.023 mol),tetrakis(triphenylphosphine)palladium(0) (1 g, 0.87 mmol).

Yield=4.27 g, (47%).

Compound 37:

3-fluoro-4-octoxy-2'-fluorobiphenyl-4'-yl boronic acid.

This was prepared using a similar method to that described for compound11. Quantities: Compound 36 (4.25 g, 0.011 mol), n-butyllithium (4. 5ml,2.5M in hexane, 0.011 mol), trimethylborate (2.22g. 0.021mol).

Yield=2.5 g, (51%).

Compound 38:

2'3-difluoro-4'-hydroxy-4-octoxybiphenyl.

This was prepared using a similar method to that described for compound12. The crude product was recrystallised from hexane to give colourlessneedles. Quantities: compound 37 (2.5 g, 5.5 mmol), hydrogen peroxide(10%, 15 ml, 0.044 mol).

Yield=1.54 g, (67%),

mp=82°-83° C.

Compound 40:

2',3-difluoro-4-octoxybiphenyl-4'-yl

2-fluoro-4-methoxycarbonyloxybenzoate.

This was prepared using a similar method to that described for compound16. The crude product was purified by flash chromatography (silica gel;petroleum ether/dichloromethane 1:2) to give a white solid. Quantities:compound 38 (0.70 g, 2.09 mmol), 2-fluoro-4-methoxycarbonyloxybenzoicacid (0.448 g, 2.09 mmol), diethylazodicarboxylate (0.438 g, 2.51 mmol),triphenylphosphine (0.659 g, 2.51 mmol).

Yield=0.78 g, (70%).

Phase Transitions (°C.) K 54.3/79.0N 174.0 I

Compound 41:

2',3-difluoro-4-octyloxybiphenyl-4'-yl

2-fluoro-4-hydroxybenzoate.

A solution of compound 40 (0.610 g, 1.15 mmol) in 20ml dichloromethaneand 20 ml ethanol was added to 35% ammonia (20 ml) at room temperature.The mixture was stirred for 4 h or until tlc indicated the reaction tobe complete. The solvents were removed in vacuo and the residue purifiedby column chromatography (silica gel; petroleum ether/ethylacetate 2:1).The product was further purified by recrystallisation from ethylacetate/hexane mixtures to give white crystals.

Yield=0.33 g, (61%),

mp=169°-170° C.

Compound 42:

2,3-difluoro-4-octyloxybiphenyl-4'-yl

2-fluoro-4-methoxycarbonyloxybenzoate.

This was prepared using a similar method to that described for compound40. Quantities: compound 12 (1.40 g, 4.2 mmol),2-fluoro-4-methoxycarbonyloxybenzoic acid (0.897 g, 4.2 mmol),diethylazodicarboxylate (0.875 g, 5.0 mmol), triphenylphosphine (1.318g, 5.0 mmol).

Yield=1.10 g, (49%).

Phase Transitions (°C.) K 60.5/117.3N 164.7 I

Compound 43:

2,3-difluoro-4-octyloxybiphenyl-4'-yl

2-fluoro-4-hydroxybenzoate

This was prepared using a similar method to that described for compound41. The product was recrystallised from ethyl acetate/hexane mixtures togive fine colourless needles. Quantities: compound 42 (1.00 g, 1.9mmol), ammonia (10-35%., 30 ml).

Yield =0.58g, (64%),

mp =179-180° C.

Compound 44:

2,3-difluoro-⁴ -octyloxybiphenyl-4'-yl

2-fluoro-4-((S)-² -fluorooctanoyloxy)benzoate.

To a solution of compound 43 (0.160 g, 0.30 mmol), (S)-2-fluorooctanoicacid (0.055 g, 0.30 mmol) and dimethylaminopyridine (0.011 g) in drydichloromethane (20 ml) under nitrogen was added a solution ofdicyclohexylcarbodiimide (0.084 g, 0.40 mmol) in dichloromethane (5 ml).The reaction mixture was stirred at room temperature for 8 h after whichthe DCU formed was removed by filtration and the solvent removed invacuo. The crude product was purified by flash chromatography (silicagel; petroleum ether/dichloromethane 1:4) and further purified by twicerecrystallising it from hexane to give a white solid.

Yield=0.10 g, (48%).

Compound 45:

2,3-difluoro-4-octyloxybiphenyl-⁴ '-yl

2-fluoro-4-((S)-2-fluorooctyloxy)benzoate

A solution of compound 43 (0.25 g, 0.53 mmol). (S)-2-fluoro-octanol(0.078 g, 0.53 mmol) and diethylazodicarboxylate (0.111 g, 0.64 mmol)was prepared in 12 ml dry THF under nitrogen. To this was added slowly asolution of triphenylphosphine (0.167 g, 0.64 mmol) in 3 ml dry THF. Thesolution was stirred overnight at room temperature. The product waspurified by flash chromatography (silica gel; petroleumether/dichloromethane 1:2) and finally recrystallised from pentane at-20° C. to give a white solid.

Yield=0.26 g, (82%).

Compound 46:

2,3-difluoro-4-octyloxybiphenyl-4'-yl

2-fluoro-4-((S)-2-fluorooctanoyloxy)

This was prepared using a similar method to that described for compound44. Quantities: compound 41 (0.201 g. 0.43 mmol), (S)-2-fluorooctanoicacid (0.069 g, 0.43 mmol), N,N-dimethylaminopyridine (0.013 g),dicyclohexylcarbodiimide (0.105 g, 0.51 mmol).

Yield=0.150 g. (57%).

Compound 47:

2,3-difluoro-4-octyloxybiphenyl-4'-yl

2-fluoro-4-((S)-2-fluorooctyloxy)benzoate.

This was prepared using a similar method to that described for compound45. Quantities: compound 41 (0.220 g, 0.47 mmol), (S)-2-fluorooctanol(0.069 g, 0.47 mmol), diethylazodicarboxylate (0.097 g, 0.56 mmol),triphenylphosphine (0.147 g, 0.56 mmol).

Yield=0.20 g, (71%)

Compound 51:

3-fluoro-4-methoxycarbonyloxybenzoic acid.

This was prepared using the method according to the published procedurein Helv. Chim. Acta 67, 1987, 1578.

Compound 52:

2,3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-4-methoxycarbonyloxybenzoate.

This was prepared using a similar method to that described for compound12. Quantities: Compound 12 (0.781 g, 2.33 mmol), compound 51 (0.50 g,2.33 mmol), diethylazodicarboxylate (0.488 g, 2.80 mmol),triphenylphosphine (0.733 g, 2.80 mmol).

Yield=0.526 g (43%)

Phase Transitions (°C.) K 84.0/103.8N 177.5 I

Compound 53:

2,3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-4-hydroxybenzoate.

This was prepared using a similar method to that described for compound41. Quantities: Compound 52 (0.50 g, 0.94 mmol).

Compound 48:

2,3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-4-(2-fluorooctyloxy)benzoate.

This was prepared using a similar method to that described for compound45. Quantities: Compound 53 (0.180 g, 0.38 mmol), (S)-2-fluorooctanol(0.056 g, 0.38 mmol), diethylazodicarboxylate (0.080 g, 0.456 mmol),triphenylphosphine (0.120 g, 0.456 mmol)

Yield=0.163 g, (71%).

Compound 49:

2,3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-⁴ -((S)-2-fluorooctanoyloxy)benzoate.

This was prepared using a similar method to that described for compound44. Quantities: Compound 53 (0.199 g, 0.42 mmol), (S)-2-fluorooctanoicacid (0.068 g, 0.42 mmol, N,N-dimethylaminopyridine (0.013 g),dicyclohexylcarbodiimide (0.104 g, 0.50 mmol).

Yield=0.165 g, (64%).

Compound 54:

2',3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-4-methoxycarbonyloxybenzoate.

This was prepared using a similar method to that described for compound45. Quantities: Compound 38 (0.715 g, 2.14 mmol), compound 51 (0.458 g,2.14 mmol), diethylazodicarboxylate (0.447 g, 2.57 mmol),triphenylphosphine (0.674 g, 2.57 mmol).

Yield=0.456 g, (40%).

Compound 55:

2',3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-4-hydroxybenzoate.

This was prepared using a similar method to that described for compound41. Quantities: Compound 54 (0.410 g, 0.77 mmol).

Yield=0.308 g, (84%),

mp=134°-36° C.

Compound 50:

2',3-difluoro-4-octoxybiphenyl-4'-yl

3-fluoro-4-((S)-2-fluorooctyloxy)benzoate.

This was prepared using a similar method to that described for compound45. Quantities: Compound 55 (0.180 g. 0.38 mmol), (S)-2-fluorooctanol(0.056 g, 0.38 mmol), diethylazodicarboxylate (0.080 g, 0.46 mmol),triphenylphosphine (0.121 g, 0.46 mmol).

Yield =0.189 g, (83%).

                  TABLE 1    ______________________________________    Phase transition temperatures    Com-  Phase Transition Temperatures/°C.    pound °K.                 S.sub.C °                         S.sub.A                              TGB.sub.A °                                    Ch    BPI  BPII I    ______________________________________    22    .      98(l)   .    .     133.4 153.3                                               154.2                                                    155.5    23    .      93(l)   .    .     141.7 162.0                                               162.7                                                    163.3    24    .      124(l)  157.4                              .      .    .    .    171.1    25    .      104(r)  156.6                              .     173.7 .    .    175.7    30    .      105(l)  .    .     141.9 155.5                                               155.6                                                    155.6    31    .      77(r)   134.5                              144.4 145.3 159.5                                               .    160.2    33    .      86(l)   .    .     120.9 136.6                                               137.4                                                    137.5    44    .      93(l)   .    .     121.1(r)                                          146.8                                               .    146.9    45    .      84(r)   .    .     113.8(r)                                          145.5                                               .    147.2    46.sup.(¶)          .      73(r)    97.2                              .     133.9(r)                                          .    .    137.7    47    .       .       38.0                              .     140.5(r)                                          .    .    144.2    48    .      102(r)  .    .     146.0 152.9                                               152.9                                                    153.8    49.sup.(¶¶)          .      80.5(l) .    .     136.6 .    .    147.0    50    .      103.5(r)                         106.5                              .      .    .    .    152.2    ______________________________________     K = Crystalline, S = smectic. Ch = cholesteric, BP = blue phase, I =     isotropic, r = righthanded twist, l = lefthanded twist, TGB.sub.A =     Twisted Grain Boundary A Phase.     Wherever possible, the source of the helicity in the chiral phases has     been determined and this is shown in parantheses after the phase     assignment, i.e. Sc*(I) represents a chiral smectic C phase with a     lefthanded helix.     .sup.(¶) For compound 46 there is a helix inversion: S.sub.C     *(r) 68.7° C. S.sub.C *(l).     .sup.(¶¶) For compound 49 there is an N.sub.∞ *     phase:     S.sub.C *(l) 136.5 N.sub.∞ * 137.0 Ch(r) 147.0 I.     N.sub.∞ * is an infinite pitch cholesteric phase (i.e. unwound) and     therefore necessarily nematic. For further details of this and related     phenomena see J. Mater. Chem., 1992, 2(8), 805-810; and J. Mater. Chem.,     1993, 3(4), 399-405.     N.sub.∞ * is where the pitch of the cholesteric phase is infinite     but the mesophase still has chiral symmetry.

An example of the use of a compound of Formula I in a liquid crystalmaterial and device embodying the present invention will now bedescribed with reference to FIG. 14.

The liquid crystal device consists of two transparent plates, 1 and 2,in this case made from glass. These plates are coated on their internalface with transparent conducting electrodes 3 and 4. An alignment layeris introduced onto the internal faces of the cell so that a planarorientation of the molecules making up the liquid-crystalline materialwill be approximately parallel to the glass plates 1 and 2. This is doneby coating the glass plates 1,2 complete with conducting electrodes 3,4with layers of film 5 and 6 of a suitable polymer, eg polyimide. Theelectrodes 3,4 may be formed into row and column electrodes so that theintersections between each column and row form an x, y matrix ofaddressable elements or pixels. Prior to the construction of the cellthe films 5,6 are rubbed with a soft tissue in a given direction, therubbing directions being arranged parallel upon construction of thecell. A spacer 7 eg of polymethyl methacrylate separates the glassplates 1 and 2 to a suitable distance eg 2 microns. Liquid crystalmaterial 8 is introduced between glass plates 1, 2 by filling the spacein between them. The spacer 7 is sealed with an adhesive 9 in a vacuumusing an existing technique. Polarisers 10, 11 are arranged in front ofand behind the cell.

The device may operate in a transmissive or reflective mode. In theformer, light passing through the device, eg from a tungsten bulb, isselectively transmitted or blocked to form the desired display. In thereflective mode a mirror (12) is placed behind the second polariser 11to reflect ambient light back through the cell and two polarisers. Bymaking the mirror partly reflecting the device may be operated both in atransmissive and reflective mode.

Tables 2-4, 6, 8-13 give values for the spontaneous polarisation (Ps)and Tables 5,7 and 14 for the tilt angle (°) over a range oftemperatures for a number of the materials described by general formulaI. The compounds of formula I are in some cases added to host materials.The following host material is given the abbreviation H1 and is a 1:1:1mixture of the following:

R₁ =C₈, R₂ =C₅

R₁ =OC₈, R₂ =C₅

R₁ =OC₇, R₂ =C₇ ##STR14##

The host H1 is a commercially available smectic host, and is widely usedin ferroelectric liquid crystal mixtures.

To measure the tilt angle a parallel 2 μm buffed polyimide (PI) coatedcell was used. After aligning the material to achieve a uniformalignment by cooling from the isotropic phase through to the Sc phase asquare wave of typically 5 V/μm at 50 Hz was applied.

Ps was measured in a 6 μm PI parallel cells, using a diamant bridge.

                  TABLE 2    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           90     1.3           85     1.4           80     1.7           75     1.9           70     2.0           65     2.5           60     2.8           55     2.9           50     3.0           45     3.3           40     3.4    ______________________________________

The data in Table 2 is for 4.6% of compound 22 in H1.

Phase Transitions (°C.) K 98 S_(c).sup.• (1) 133.4 Ch 153.3 BPI 154.2BPII 155.2 I

                  TABLE 3    ______________________________________           Temp/°C.                  ps/nC/cm.sup.2    ______________________________________           100    1.5           95     3.0           90     4.0           85     5.0           80     5.5           75     6.0           70     7.0           65     7.4           60     8.2           55     8.6           50     9.1           45     9.6           40     10.2           35     10.6    ______________________________________

The data in Table 3 is for 10% of compound 22 in H1. Phase Transitions(°C.): S_(c) 101.9 Ch 151.1 I

                  TABLE 4    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           95     6.0           90     6.9           85     9.5           80     10.5           75     11.0           70     11.5           65     12.7           60     13.8           55     14.0           50     16.7           45     17.4           40     18.1    ______________________________________

The data in Table 4 is for 15% compound 22 in H1.

Phase Transitions (°C.): S_(C) 99.4 CH 149.3 I

                  TABLE 5    ______________________________________           Temp/°C.                  tilt angle/°    ______________________________________           100    17           95     21           90     20           85     19           80     19.5           75     18.5           70     17.5           60     17.5           50     17.0           40     17.5    ______________________________________

The data in Table 5 is for 5% compound 22 in H1.

                  TABLE 6    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           130    49.3           125    55.6           120    61.8           115    65.7           110    70.8           105    74.7           100    85.3           95     89.4           107    80.0    ______________________________________

The data in Tables 6 and 7 is for compound 23.

Phase Transitions (°C.): K 93 S_(C).sup.• (r) 141.7 Ch 162.0 BPI 162.7BPII 163.3 I.

                  TABLE 7    ______________________________________           Temp/°C.                  tilt angle/°    ______________________________________           130    24           125    25.5           120    18           115    17           110    18           105    17           100    18           95     18    ______________________________________

                  TABLE 8    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           95     1.8           90     1.9           85     2.0           80     2.1           75     2.1           70     2.1           65     2.0           60     1.66           55     1.65           50     1.43           45     1.32           40     1.28    ______________________________________

The data in Table 8 if for 10% of compound 23 in H1.

Phase Transitions (°C.): S_(C) 153.0 I

                  TABLE 9    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           150    44.2           145    68.3           140    84.5           135    95.9           130    107.0           125    117.3           127    113.1    ______________________________________

The data in Table 9 is for compound 24

Phase Transitions (°C.): K 124 S_(C).sup.•(1) 157.4 S_(A) 171.1 I

                  TABLE 10    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           150    17.1           148    21           146    24.2           144    28.8           135    35.4           130    38.8           125    42.0           120    44.6           115    47.2           110    49.5           105    51.5    ______________________________________

The data in Table 10 is for compound 25

Phase Transitions (°C.): K 104 S_(C).sup.•(r) 156.6 SA 173.6 TGB A*173.8 Ch 175.7 I

                  TABLE 11    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           135    85           132    85           130    95           125    105           120    115           115    125           110    135           105    143    ______________________________________

The data in Table 11 is for compound 30

Phase Transitions (°C.): K 105 S_(C).sup.• (1) 141.9 Ch 155.5 BPI 155.6BPII 155.6 I

                  TABLE 12    ______________________________________           Temp/°C.                  ps/nC/cm.sup.2    ______________________________________           136    7.5           130    24           126    30           120    37           116    41           110    45.5           106    48           100    --           96     56           90     60           86     63.5           80     65.5           76     66.5           70     69           66     70    ______________________________________

The data in Table 12 is for compound 31

Phase Transitions (°C.): K 77 S_(C).sup.• 134.5 S_(A) 144.4 TGB A* 145.3Ch 159.5 BPI

                  TABLE 13    ______________________________________           Temp/°C.                  Ps/nC/cm.sup.2    ______________________________________           116    1.2           110    4.0           106    5.0           100    5.5           96     5.8           90     6.4           86     6.6           80     6.6           76     6.7    ______________________________________

The data in Table 13 is for compound 33

Phase Transitions (°C.): K 86 S_(C).sup.• (1) 120.9 Ch 136.6 BPI 137.4BPII 137.5 I

                  TABLE 14    ______________________________________           Temp/°C.                  tilt angle/°    ______________________________________           140    18.5           135    26.5           130    29.5           125    30.0           120    30.5           110    32.0    ______________________________________

The data in Table 14 is for compound 48.

We claim:
 1. A compound having a formula I ##STR15## in which R isselected from alkyl, alkoxy or alkenyl and contains 1-20 carbon atoms;A,B, C, D are independently selected from phenyl, cyclohexyl, pyridyl,pyrimidyl; b is independently selected from 0, 1 or 2; c isindependently selected from 0 or 1 provided that the total of b+c is notgreater than 2; K, L, M, N are independently selected from the halogengroup; (k), (1), (m), (n) are independently selected from 0, 1, 2, 3 or4; provided that at least one of the groups A, B, C, D present is aphenyl group with at least one fluorine substituted on it; X is selectedfrom OCO, COO, OCH₂, CH₂ O, CH₂ CH₂ ; R₂ is selected from the following:##STR16## R₃ is a linear alkyl group containing 1-15 carbon atoms; R₂may also be the following: ##STR17## R₄ is a linear or branched chainalkyl group containing 1-15 carbon atoms provided that when R₂ isselected from ##STR18## then b+c=1 or 2 and A, B, C, D groups are phenylgroups.
 2. A compound according to claim 1 wherein whichever of A, B. C,D are present are selected from phenyl.
 3. A compound according to claim2 wherein X is COO or OCO.
 4. A compound according to claim 3 whereinwhichever of K. L, M, N present are selected from fluorine.
 5. A liquidcrystal device comprising a layer of liquid crystal material containedbetween two spaced cell walls each bearing electrode structures andsurface treated on facing surfaces to align liquid crystal materialmolecules, characterised in that the liquid crystal material includesthe compound as described in claim
 1. 6. A liquid crystal mixturecontaining any of the compounds of claim 1 and a host material of thefollowing general formula: ##STR19## where R₁ and R₂ are independentlyC₃ -C₁₂ alkyl or alkoxy.
 7. A liquid crystal mixture containing any ofthe compounds of claim 1 and a host material of the following generalformula: ##STR20## where R₁ and R₂ are independently C₃ -C₁₂ alkyl oralkoxy x is 1 and F may be on any one of the available substitutionpositions on the phenyl ring specified.
 8. A liquid crystal mixturecontaining any of the compounds of claim 1 and a host material of thefollowing general formula: ##STR21## where R₁ and R₂ are independentlyC₃₋₁₂ alkyl or alkoxy.
 9. A liquid crystal mixture containing any of thecompounds of claim 1 and a host material of the following generaformula: ##STR22## where R₁ is C₃₋₁₂ alkyl and R₂ is given by thegeneral formula (CH₂)_(n) -CHXCH₂ CH₃, where n is 1 to 5 and X is CN orCl.
 10. A liquid crystal mixture containing any of the compounds ofclaim 1 and a host material of the following general formula ##STR23##where R₁ and R₂ are independently C₁ -C₁₅ alkyl.
 11. A liquid crystalmixture containing any of the compounds of claim 1 and a host materialof the following general formula: ##STR24## where R₁ and R₂ areindependently C₃ -C₉ alkyl.
 12. A device according to claim 5 includinga host material of the following formula: ##STR25## where R₁ and R₂ areindependently C₃ -C₁₂ alkyl or alkoxy.
 13. A device according to claim 5including a host material of the following formula: ##STR26## whereR.sub. and R₂ are independently C₃ -C₁₂ alkyl or alkoxy, x is 1 and Fmay be on any one of the available substitution positions on the phenylring specified.
 14. A device according to claim 5 including a hostmaterial of the following formula: ##STR27## where R₁ and R₂ areindependently C3-C₁₂ alkyl or alkoxy.
 15. A device according to claim 5including a host material of the following formula: ##STR28## where R₁is C₃ -C₁₂ alkyl and R₃ is given by the formula (CH₂)_(n) --CHXCH₂ CH₃,where n is 1 to 5 and X is CN or Cl.
 16. A device according to claim 5including a host material of the following formula: ##STR29## where R₁and R₂ are independently C₁ -C₂ alkyl.
 17. A device according to claim 5including a host material of the following formula: ##STR30## where R₁and R₂ are independently C₃ -C₉ alkyl.